Wireless communication, and indeed nearly all communication, has as a significant goal the maximization of the information that can be transmitted over a particular communication channel. Maximization of information transfer is of particular interest in wireless communication channels in part because wireless channels are subject to constraints and difficulties that are less prevalent with wired channels.
In wireless and other communication channels, information is typically transmitted from a transmitter to a receiver in the form of signals transmitted over a wireless channel. In order to increase the amount of information that can be transmitted over a channel, as well as to provide for error correction, information is transmitted not in the form of bits, but instead in the form of symbols representing bits. In one common system of encoding, a bit stream is encoded using a channel code that adds redundancy and error correction, and the encoded bit stream is mapped to a sequence of complex-valued symbols selected from a constellation, which are transmitted as modulations of a carrier wave. At the receiver, the symbols must be detected and decoded in order to identify the symbols being received and to determine the information content being received. The encoded bit stream must be reconstituted from the symbols and the original bit stream must be recovered from the encoded bit stream. The choice of encoding and decoding techniques is dependent on a number of factors, including the need to transmit information efficiently, the need to detect and correct errors and the need to maximize computational efficiency in encoding and, in particular, in decoding the symbols.
One particularly advantageous way to achieve high data transfer rates on a scattering-rich wireless channel is to use multiple transmit and receive antennas. Many of the practical space-time methods that achieve high data transfer rates are designed to use simple symbol detection at the receiver because they map the symbols linearly to the transmit antennas. Different coding techniques employ different performance criteria. For example, an encoding technique may be designed to optimize a raw block or bit pairwise error performance criterion, while another technique may be designed to optimize an information-theoretic criterion.
Whichever technique is used, any technique designed to achieve capacity on a channel will typically require some form of “outer” channel code that provides redundancy, and interleavers to guard against bursty fading, interference and additive receiver noise. In such a case, a space-time encoder used to encode the information content of the symbols to be transmitted will construct an “inner” code that encodes the symbols. An “outer” code will introduce redundancy, and the transmitted symbols will represent the redundancy encoding as well as the information content. A space-time detector at the receiver will be required to detect and decode symbols that are correlated through the channel code, thus significantly complicating the detection process. That is, the space-time detector will be required to determine the information content of the symbols received through the channel, taking into account the fact that the symbols include outer coding to add redundancy and error correction as well as inner coding representing the information content.
The space-time encoding and the channel encoding can be combined. An example of an encoding system employing the combination of space-time encoding and channel encoding are trellis codes. However, these combined codes are typically designed by hand and have state complexity that increases exponentially with the number of antennas used. This rapid increase in complexity greatly increases computational requirements imposed by the use of such codes in a multiple antenna environment.
There exists, therefore, a need for encoding and decoding techniques that will operate suitably for increasing numbers of transmit and receive antennas, and at the high transmission capacities achievable by these antennas on a flat-fading channel.